In semiconductor fabricating steps, a pattern dimension of a semiconductor device is measured, and its finishing result is evaluated by using a charged particle beam apparatus of a scanning electron microscope (Scanning Electron Microscope, hereinafter, referred to as SEM) or the like. In particular, an SEM which measures a line width of a line pattern or a hole diameter of a hole pattern of a semiconductor is referred to as CD-SEM (Critical Dimension SEM).
In CD-SEM, an electron beam which is emitted from an electron gun that is installed upward from a wafer is focused to slenderize by a convergent lens, and is two-dimensionally scanned on a sample to be evaluated by a deflector. A secondary electron which is generated from a surface of the sample by irradiating the surface with the electron beam is caught by a secondary electron detector, and an obtained signal is recorded as an image (hereinafter, referred to as an SEM image). An amount of generating the secondary electron is changed by recesses and protrusions of the sample. Therefore, a change in a shape of the surface of the sample can be grasped by evaluating a secondary electron signal. An edge position is specified by using the fact that the secondary electron signal is rapidly increased at an edge position of a pattern, and a dimension is measured (hereinafter, referred to as length measurement). However, in a case where a resist material is measured by CD-SEM, there pose two problems which become obstacles. First, a photoresist for argon fluoride (ArF) excimer laser which is used as a photosensitive material of photolithography (hereinafter, referred to as ArF resist) is fragile at an electron beam and a volume thereof is reduced by an electron beam irradiation (hereinafter, referred to as shrinking).
Second, there occurs an electrostatic charge of a surface of a sample which is caused by irradiating an electron having a negative electric charge. The electrostatic charge of the surface of the sample by an electron beam irradiation can be represented by a secondary electron emission coefficient δ which is defined by (secondary electron amount/incident electron amount), and shows a characteristic as shown in FIG. 1. An irradiation energy is partitioned to three ranges (101, 102, 103) with an irradiation energy at which numbers of particles of incident electrons and secondary electrons coincide with each other, that is, an irradiation energy at which the second electron generating efficiency δ becomes 1 as a boundary. At ranges 101 and 103, the number of particles of the secondary electrons is smaller than the number of particles of the incident electrons, and the sample is electrically charged negatively. In contrast thereto, at range 102, the number of particles of secondary electrons is larger than the number of particles of incident electrons. Therefore, the sample is electrically charged positively. When the sample is electrically charged positively at several V, the secondary electron having an energy as small as several eV is pulled back to the surface of the sample, and also the incident electrons are deflected. As a result thereof, a brightness of an SEM image is darkened, a dimensional error by deflecting the incident electrons is brought about, and it seems that its influence is effected on the measurement.
Patent Literatures 1 and 2 disclose a method of FIG. 2 in order to presume a dimension before changing the dimension by shrinkage. A length of a dimension of a pattern which is shrunk by an electron beam irradiation is repeatedly measured, a result of the length measurement is plotted (201), and an approximate expression (202) is fitted to the plotting (201) of a corresponding length measurement value. Thereby, dimension before irradiating an electron beam, that is, a dimension (203) before having been shrunk is presumed. However, according to the method, no mention is given to an influence of an electrostatic charge, and the method is on the premise that a shrinking amount stays the same at all portions of a resist pattern which is an object.
Patent Literature 3 describes a technology of carrying out a highly reliable dimension measurement by extracting information of a contour based on an actual pattern edge end of a sample.